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The present invention relates to magnetic position encoders, and more particularly to magnetic position encoders utilizing mechanically shaped, magnetic field generating disks to optimize the resulting magnetic field pattern.
Many applications require instantaneous information regarding the linear or angular position of one element relative to another. For example, it is often desirable to measure the position of a rotating shaft with respect to a fixed reference, such as a stator holding the shaft bearings. Devices that measure such an angular position are well known in the art and include resolvers, optical encoders, magnetic encoders, rotary potentiometers, and others. Such devices, known for example as position encoders or position sensing devices, all have various economic and performance trade-offs. In particular, the least expensive high resolution position sensing devices are typically incremental as opposed to absolute. An incremental position sensing device is characterized by the fact that it only produces positional information regarding a movable member in the form of a quantized change from the previous position. The resolution of the device is defined by the smallest quantum of positional change that the sensing device can indicate (i.e., an angular increment for rotational sensing device or lineal increment for a linear sensing device). Such devices typically incorporate associated electronics that require external power. Once power is applied to the associated electronics of an incremental position sensing device, the device can begin producing incremental position information. By utilizing appropriate interface electronics (e.g., an accumulator), such sensors can keep track of how far the movable member has traveled since external power has been applied, but the absolute position is unknown, because the device has no information regarding the position of the movable member at the time the power was applied.
One type of prior art absolute position sensing device operates by producing a magnetic field that is fixed relative to the movable member, and sensing the magnetic field at a fixed reference position (relative to the movable member) as the movable member travels through its range of motion. In one such absolute position sensing device for sensing the angular position of a rotatable shaft, the magnetic field is generated by a permanently magnetized disk, fixedly mounted to the shaft. The fixed sensor produces a discrete output at the maxima of the detected magnetic field. Ideally, the field variation detected by the fixed sensor is sinusoidal, with the maxima occurring at the poles of the magnetized disk. One disadvantage to such a sensing device is that the sinusoidal field detected at the fixed sensor tends to be distorted, i.e., the detected waveform includes higher order harmonics, in addition to the desired fumdamental sinusoid. This distortion introduces an error in the estimate of position.
It is an object of this invention to provide a position encoder that substantially overcomes or reduces the aforementioned disadvantages while providing other advantages which will be evident hereinafter.
The present invention is an apparatus for indicating the instantaneous position of a movable member, movable along a range of motion with respect to a reference position. The apparatus includes a magnetic body that is fixedly attached to the movable member. The body has a contoured surface and produces a magnetic field. The apparatus further includes at least two magnetic field sensors. Each of the sensors is disposed adjacent to the contoured surface of the magnetic body, fixed with respect to the reference position, and is positioned apart from the other sensors, so as to form a non-uniform gap between the contoured surface and each of the magnetic field sensors. Each of the sensors produces a position dependent signal corresponding to the value of the magnetic field. The apparatus further includes a decoding circuit electrically coupled to each of the magnetic field sensors so as to receive each of the position dependent signals. The decoding circuit produces a position dependent signal that is a predetermined function of the magnetic field sensor position dependent signals.
In one preferred embodiment, the magnetic field sensors include at least two sets of two magnetic field sensors, so as to provide electrically redundant position dependent signals.
In yet another embodiment, the gap is tailored to vary as a periodic non-uniform function between gmax and gmin and having an inverse sinusoidal component.
In another embodiment, the non-uniform gap is tailored so as to produce a sinusoidal variation with the position of the movable member with respect to the reference point.
In one embodiment, the magnetic body is selected from the group consisting of a permanent magnet, an electromagnet, a high mu material, a combination thereof, or a combination thereof with non-magnetic materials.
In another embodiment, the magnetic body is rotatable and includes N magnetic poles, where N is preferably an integer evenly divisible by four so that redundant magnetic field sensors can be incorporated.
In another embodiment, the movable member and the magnetic body are combined so as to include a single magnetic member.
In another embodiment, each of the magnetic field sensors further includes at least one adjustment screw disposed substantially adjacent to the magnetic field sensor so as to interact with the magnetic field. The position dependent signal amplitude varies as a predetermined funtion of a relative distance from the magnetic field sensor to the adjustment screw. The adjustment screw includes a high magnetic permeability material.
In another embodiment, the magnetic field sensors include linear Hall effect sensors.
In another embodiment, the movable member includes a shaft rotatable about a rotation axis, and a disk having an outer periphery defined by a non-uniform radius. The disc is fixedly attached to the shaft at the disc center such that the disk rotates within a plane substantially normal to the rotation axis. The magnetic body is fixedly attached to the disk, and the magnetic field sensors are disposed about the outer periphery at substantially equal radii with respect to the rotational axis.
In another embodiment, the apparatus further includes two sets of three magnetic field sensors disposed at angular intervals substantially equal to 120 degrees.
In another embodiment, the movable member includes an elongated object movable along a linear axis. The elongated object has at least one exterior surface substantially parallel to the linear axis, and the magnetic field sensors are disposed along the exterior surface at substantially equal radial distances with respect to the linear axis.